Shape memory polymer (SMP) is a kind of programmable smart material. Upon external stimuli, SMP could recover from a temporary shape to an original or a permanent shape. SMPs have been widely utilized as heat shrink tubes for cable industry and heat-shrink labels for packaging industry, and the later use achieved high level automation in traditional packaging industry.
Conventional SMPs could merely transform from one temporary shape to one permanent shape, and thus defined as dual-shape SMPs. The ability to lock the temporary shape lies in the phase transformation. Lendlein et al proposed a polymer material that undergoes two distinct phase transformation, each of which could fix an independent temporary shape and therefore achieved triple-shape memory effect (Lendlein, Proc. Natl. Acad. Sci. USA, 2006, 103, 18043). Xie Tao further discovered phase transformation with a broad transition temperature range, which could be tuned to fix more than two temporary shapes and accomplished multi-shape memory effect (Xie, Nature, 2010, 464, 267). These shape memory polymers mentioned above could recover to the permanent shape from the temporary shape upon heating, but never return to the temporary shape upon cooling without external force present. The irreversibility of the shape transformation categorizes those materials as one-way SMPs.
Progress in reversible shape memory polymer or two-way shape memory effect has been made remarkable progress in recent years. Several research groups reported the reversible shape change, that is cool-induced elongation and heat-induced contraction, which has been known for mono-domain liquid crystalline elastomer with thermal treatment around the liquid crystal transition temperature, in crosslinked semicrystalline polymer. However, the reversible shape changes are confined to simple contraction or bending and thus the accessible shapes could not be arbitrarily programmed. Lendlein et al developed a polyurethane with two distinct crystalline phases in 2013. The crystalline phase with a lower transition temperature acted as the shape shifting phase and the other was designed for altering the permanent shape for during reversible change cycle and programmability was thus achieved (Lendlein, Adv. Mater., 2010, 22,3424).
However, the SMPs discovered so far are intrinsically compromised for their limited shape complexity, since the permanent shapes of the SMPs are dominantly determined by the processing mold, the cost of which would be extremely high for achieving complicated shapes. Moreover, the required chemical crosslinks for SMPs deprive them of the possibility for post-fabrication like thermoplastic polymers. Regular thermoplastics or thermosets would neither suffice for circumstances that multi-step shape modification or accumulated hierarchical shape is necessary.